This case-control study addresses the hypothesis that dietary cofactors are involved in the etiology of breast cancer. Most previous epidemiologic studies have concentrated on fats in relation to breast cancer or have considered only a few other nutrients. While the experimental literature indicates that fats are positively associated with breast cancer risk, it also suggests that vitamins A, C and E, and zinc, selenium, fiber and protease inhibitors are protective against breast cancer, and that certain joint or interactive effects among these nutrients are important to take into account. This study will address these hypotheses in a human population. The subjects in this ongoing study will be about 400 breast cancer cases and 600 controls obtained from a breast cancer screening program over a six year period. Information will be obtained on other risk factors for breast cancer. An extensive dietary questionnaire is being administered by telephone, and from this nutrient indexes will be calculated. Special methodological sub-studies will be performed to assess the reliability and validity of the dietary questionnaire. Analyses will use multivariate methods to control for confounding factors and to permit an examination of interactions between various dietary factors in breast cancer etiology. This study should provide leads for more focused observational or intervention studies of the role of diet in breast cancer prevention.